It is proposed to characterize and localize the two interdependent processes of Cl-minus absorption and HCO3-minus secretion in in vitro proximal segments of normal Amphuima small intestine as well as the enhanced HCO3-minus secretion in the presence of theophylline in the following ways: 1) The involvement of Na ion in the process will be examined further by determining the effect of substituting one of several cations for Na ion on mucosal membrane potential and membrane input resistance as well as intracellular Cl-minus activity and net Cl-minus absorption. For this purpose conventional microelectrodes, Cl-minus-sensitive liquid ion-exchanger microelectrodes, and the short-circuiting technique will be used. In this way the extent to which Na-Cl co-transport is involved in energizing Cl-minus accumulation will be given fuller consideration. 2) Localization of the Cl-minus (and Na ion) absorptive process will entail the use of a new chamber which allows measurment of unidirectional Cl-minus and Na ion fluxes in isolated villus and intervillus regions of the mucosa. Also intracellular Cl-minus activity will be measured in the intervillus absorptive cells. This will show whether electrogenic Cl-minus transport in the villus alone accounts for Cl-minus absorption in the whole mucosa. 3) Intracellular HCO3-minus activity will be measured with HCO3-minus sensitive liquid ion-exchanger microelectrodes and intracellular pH with DMO. These will provide two independent measures of the cytoplasmic HCO3-minus concentration under different conditions designed to test whether this variable has a controlling influence on the mucosal Cl-minus uptake process and the relationship between cytoplasmic HCO3-minus concentration and net HCO3-minus transport in normal and theophylline-treated intestine.